Flipping keyframes in walk cycle animation not working
Something weird is going on, but you can solve it by selecting in edit mode all the .L bones, delete them, then select all the .R bones, Shift+D to duplicate, Esc not to move them, Shift+C set the cursor to center, make the cursor pivot point, Ctrl+M  X to mirror the bones on the X axis, Then Armature - Flip Names.

Everything now works as expected.

The reverse (L-R) of this whole operation doesn't work!